At present elongation after rupture is determined in the computer controlled tensile test (to DIN EN 10002/1) using an extension sensor attached to the tensile testpiece by two measuring sensors. Preferably incremental, but also inductive extension sensors are used. The extension sensors measure the elongation of the testpiece between the two measuring sensors until the testpiece ruptures. The accuracy of the measuring result mainly depends on the position of the rupture in relation to the two measuring sensors. Elongation after rupture can be reliably measured in this manner only if rupture takes place in the center between the measuring sensors. In the case of highly ductile materials, even eccentric ruptures which still clearly lie between the measuring sensors result in lower values of elongation after rupture, since a proportion of the elongation with necking lies outside the sensors and is therefore not measured by the extension sensor.
The determination of the elongation after fracture is improved if the extension of the tensile testpiece decisive therefor is determined as the sum of elongation without necking and elongation with necking. The elongation with necking corresponds to the total elongation of the testpiece (main transverse path) after the maximum force has been exceeded. However, that method provides reliable values, independently of the position of the rupture in relation to the measuring sensors, only if the measuring length L.sub.o (L.sub.o =initial measuring length (mm) of the tensile testpiece prior to the tensile test) is substantially equal to the testpiece test length L.sub.c (mm). With an increasing test length in relation to the measuring length (increasing L.sub.c /L.sub.o ratio) increasingly higher and therefore incorrect values of elongation after fracture are obtained in the computer controlled tensile test. The reason for the incorrect values of elongation after fracture is that in that method the whole extension with necking is always measured. In contrast, clear values of elongation after rupture are obtained even in the case of eccentric ruptures and end ruptures when elongation after rupture is determined manually to DIN EN 10002/1 (Journal: Materialprufung 31 (1989) 11-12, pp. 371-376).
For the measurement of the elongation of a tensile testpiece, but not its elongation after rupture, an apparatus is known in which there are disposed on the tensile testpiece two spaced-out lines which extend transversely of the pulling direction and which are scanned together with fixed comparison markings by one or two line sensing cameras. The elongation of the tensile testpiece can be determined from the measured values thus obtained (DE 38 13 340 A1). That apparatus is not suitable for determining the elongation after fracture of a tensile testpiece.